Thanks to http://www.spatialreference.org/ , it is very easy to check the WKT definitions for WGS84 and "Google Mercator" projection. But the actual mathematical expression to convert Google Mercator to WGS84 is not so easy to check, although the expression is published at http://en.wikipedia.org/wiki/Mercator_projection#Mathematics_of_the_projection . This entry shows how we can check the express